A basic physical from a general practitioner can only treat and tell basic things. Even a detailed blood test can only show levels like thyroid, vitamin d and b and even test testosterone levels. Even if they're low doesn't mean the individual may even have dysthymia or not. Obviously, to get a proper diagnosis of bipolar, only a psychiatrist can diagnose if someone is bipolar or not. And only a psychiatrist can properly treat someone with bipolar spectrum disorders.